From Miller and others (1998): "Aniakchak Crater is an ice-free, circular caldera about 10 km in diameter and a maximum of 1 km deep which was first described by Smith (1925). The pre-caldera cone was built upon a basement of Tertiary sedimentary and volcanic rocks and Jurassic-Cretaceous sedimentary rocks, which are exposed high on the east and south walls of the caldera (Detterman and others, 1981). The elevation of the caldera rim varies from 1,341 m to 610 m. Surprise Lake, a 3.2-km-long lake in the northeast part of the caldera at an elevation of about 335 m is the source of the Aniakchak River, which flows through a breach in the eastern wall of the caldera. Numerous domes, flows, and cones occupy the interior of the caldera (Neal and others, 1992); the largest cone is Vent Mountain, 2.5 km in diameter and rising 430 m above the floor of the caldera. The pre-caldera cone was built on the west side of a basement high. The cone was deeply dissected by numerous glaciers that cut U-shaped valleys into the slopes before the caldera-forming eruption.

"Ash flows from the caldera-forming eruption - 3430 +/- 10 yrs B.P. (Miller and Smith, 1987) - reached both the Bering Sea and the Pacific Ocean (Miller and Smith, 1977). They are typically non-welded and fill glacial valleys to a depth of at least 75 m adjacent to the caldera rim. The ash flows were highly mobile, over-running 260-meter-high passes in the Aleutian Range and traveling as far as 50 km from the caldera rim (Miller and Smith, 1977)."